The present invention relates to a process and an apparatus for promoting gas-liquid reaction in a reactor vessel. It relates particularly to introducing oxygen to liquid medium of fermentation and dissolving oxygen into wastewater for aerobic treatment of sewerage.
Contacting of a gas with a liquid for the purpose of mass transfer and promoting of chemical reaction is a very important unit operation in chemical industry. The reaction may take place with or without the presence of a solid phase as a reactant or as a product. In the fermentation process the provision of oxygen and temperature control of the reaction are the most important factors, in which oxygen is dissolved in the liquid and subsequently supplied to the microbial cells to maintain their growth. Meanwhile, the reaction heat must be removed by heat exchange process for keeping the reaction at an optimum reaction temperature in order to obtain the best yield. Similar principle applies to wastewater biological process. The oxygenation of wastewater supplies oxygen to the biomass of the reactor liquid which promotes the growth of sludge and results in bacterial decomposition of the dissolved organic impurities.
A common known manner of mixing a gas with a liquid is to introduce gas under pressure near the bottom of the reactor and to permit the injected gas to bubble up through the liquid. Submerged diffusers are generally used for breaking up the gas into fine bubbles and distributing them more or less homogeneously over the reactor. Gas is transferred by the continuous contact of the fine bubbles with the liquid. In this method the mixing is limited by the gas compression power, it is therefore not very efficient in the sense of gas dissolution per unit power consumption due to lack of turbulent mixing in the liquid phase. Furthermore, the mechanical system involves extensive pipe lines, distribution manifolds, diffusers and expensive gas compressors and gas filtration system. It therefore requires high capital costs and high power cost. In addition, the maintenance of this system is very troublesome due to the frequent plugging of the diffusors, especially when the reaction is in presence of solids such as industrial fermentation and aerobic wastewater treatment.
Another known method for gas-liquid mixing, so called submerged agitating system, uses mechanically rotational agitators to intensify the turbulent mixing in the reactor liquid. Pressurized gas is introduced to the effective agitating area through pipe openings or through spargers in the form of coarse bubbles which are then broken up into small bubbles by the mixing power of the a mechanical agitator. The efficiency with which the power transmitted through mechanical agitator is generally greater than that with which it is transmitted through gas compression. Agitation also increases the residence time of the bubbles in the liquid and such prolonged residence time is very beneficial for gas-liquid mass transfer. However, the advantage of higher efficiency is somehow counterbalance by the relative complications of the mechanical system which includes submerged agitators, driving motors, gear reducers, long shafts, shaft seals in addition to the expensive gas compression system.
A common disadvantage of the above methods is that the reacting gas can be only partially dissolved due to the limitation of the gas residence time in the liquid. This characteristic is particularly undesirable when the reacting gas component is only slightly dissolvable or when the accompanied reaction is slow. For example, oxygen soluablity in water is only about 10 part per million at normal temperature. A submerged diffusor system can in general dissolve about 10% injected oxygen in water while a submerged agitating system can achieve about 20% oxygen absorption. In order to utilize the reacting gas more efficiently and more economically, gas must be recollected from the gas space of the reactor and recompressed before recirculating to the body of liquid. If the gas contains reactive, corrosive or explosive components, the process of gas recompression may be very difficult and costy. For example, the contact of high purity oxygen with compressor lubricant can couse dangerous explosion. As a consequence, specially designed oxygen compressor are required. The difficulty of the compressor design not only increases the plant capital cost but also damages the process reliability. In fact this is the main reason that the submerged diffuser system is not general applied to oxygen wastewater treatment.